1. Field of Invention
This invention relates to track while scan radar systems, and more particularly to an improved track while scan operation on scintillating point targets.
2. Description of the Prior Art
The principles of operation of track while scan radar systems are well known in the art. These systems scan through a given sector and provide instantaneous information on the location of a designated target within the scanned sector by determining the range and relative azimuth angle of the target from the radar antenna. Through the use of "sliding window" computer processing, the system tracks the target while the scanning of the entire sector area continues. The system performs the target tracking operation through a comparison of the reflected target energy of the return pulses taken with respect to timing gates in an azimuth and range timing axis, and provides coordinate values for an azimuth cursor and a range cursor. The azimuth cursor coordinates nominally represent the azimuthal point which bisects the energy distribution of a target envelope generated by the locus of the magnitudes of all of the return pulses received along the azimuth axis during a single scan through the target, while the range cursor coordinates represents the range value coinciding with the midpoint of the energy distribution of each individual return pulse. Typically, the track while scan radar narrows down the location of the target to one or two pulses in azimuth and to one range gate resolution in range. All this is known in the art, and an illustration of the tracking function detail is shown in my U.S. Pat. No. 3,845,481. An early, analog track while scan radar of this type is disclosed, inter alia, in Frank, U.S. Pat. No. 3,182,320, assigned to the U.S. Air Force. More sophisticated, digital track while scan radar systems are exemplified by the systems employed by the FAA for aircraft traffic control.
In the prior art track while scan systems, the operation is ideally based upon an assumed smooth return from the target, where all of the generated main bang pulses are reflected and received by the radar receiver, and are distributed in a coherent manner. However, the effects of scintillation, caused partly by small pockets or strata of air whose temperatures and densities differ slightly from those of their surroundings, are not accounted for. These scintillation effects result in distorted beam transmission which produces rapid variations in return pulse amplitude, creating an apparent change in target position or shape. For a moving target the effects of scintillation create difficulties and inaccuracies in determining the midpoint of the azimuth energy distribution since the scintillation effects differ in subsequent scans through the target as a result of the change in relative antenna to target azimuth position. Therefore, in any single scan of a scintillating target return signal the comparison of the energies within the target envelope in the azimuth left and right gates may be quite misleading and result in large short term errors.